Cell immunoisolation is a procedure which involves the placement of the cells or cell clusters within a semipermeable membrane barrier prior to transplantation in order to avoid rejection by the immune system. It can be applied to all cell types secreting a bioactive substance either naturally or through genetic engineering means. In practice, the main work has been performed with insulin secreting tissue. The molecular weight (Mw) cut-off of the encapsulating membrane can be controlled by the encapsulation procedure so as to exclude inward diffusion of immunoglobulins and lytic factors of the complement system, but allow the passage of smaller molecules such as glucose and insulin. The barrier permits therefore the .beta. cell to respond physiologically to changes in blood glucose but prevents any contact with components of the immune system. Under these circumstances, xenogeneic tissue could be used, thus eliminating the supply problem, and no immunosuppression would be required to prevent rejection or disease recurrence since the grafted islets would be isolated from the host's immune system.
Early studies, which explored the immunoisolation principle using diffusion chambers to enclose islet tissue or pancreatic fragments, met with little success (reviewed in 1, 2). While transitory amelioration of hyperglycaemia was attained, available membrane materials did not permit prompt stimulus/secretion transport of insulin (3). More recently, the use of hollow capillary fibres in conjunction with allogenic or xenogeneic islets enclosed within a semipermeable chamber as an extracorporeal or intravascular insulin-secreting device has been successfully used for short term reversal of diabetes in rodents (4,5), dogs (6,7) and monkeys (8). The extracorporeal or intravascular approaches, although essential to prove the soundness of the encapsulation technique, are not fit for human applications especially in young children. Diffusion chambers--the method of choice for human applications--are, however, still hampered by consistency problems (9).
Several polymer capsule fabrication methods, based on different engineering techniques, have been developed. Encapsulation procedures are most commonly distinguished by their geometrical appearance, ie micro- or macro-capsules. In macro-encapsulation, cells or cell clusters are encased within permselective hollow fibres or flat sheet membranes. Since they are fabricated from thermoplastics, these capsules are mechanically stable and relatively easy to retrieve. Several investigators have reported the successful use of the thermoplastic based hollow fibre capsules to transplant islet cells in rodent models of diabetes. We have previously reported that, given appropriate surface microgeometry and chemical composition, the tissue reaction formed around implanted thermoplastic-based macrocapsules is minimal in both the brain (10) and the peritoneal cavity (11,12) of rodents. We have also reported long-term brain survival of macroencapsulated PC12 cells, a dopaminergic cell line, when transplanted across species (13) and that these implants significantly ameliorate behaviours in rat and primate (14) experimental Parkinsonian models. Using the same encapsulation system, Lacy and collaborators have reported the correction of streptozotocin-induced hyperglycaemia in rats implanted with subcutaneous macroencapsulated islet cells (15). More recently Scharp and collaborators have reported the 2 week survival of encapsulated human islets in diabetic patients using the same acrylic-based macro-encapsulation system (16). Using a similar acrylic system, we have recently reported the successful transplantation of bovine chromatin cells in the intrathecal space of humans suffering from terminal cancer pain. Explanted devices showed an absence of host reaction to the capsule as well as viable chromaffin cells. At retrieval, the capsules released catecholamine amounts comparable to those measured in vitro prior to transplantation. Although mechanically stable and biocompatible, hollow fibre based systems require a low packing density to allow for proper viability of the transplanted cells. The requirement to scale up this material system to correct diabetes in a human would require an impractical 50 m long device. Another limitation of this technique is the thickness of the capsule wall and its potential influence on glucose diffusion kinetics. The diffusion barrier may incur short-term hypoglycemic episodes due to excessive insulin secretion.
We have demonstrated that macroencapsulation using semipermeable hollow fibres is a viable technology for the xenogeneic transplantation of endocrine tissue in humans. Although this technology has also been used experimentally for the encapsulation and transplantation of islets, it is not appropriate for their effective packaging. The wall thickness of the capsules are usually a minimum of 100 .mu.m and in the hollow fibre the cells are immobilized within a hydrogel matrix core typically 500-600 .mu.m in diameter. This creates diffusion distances of several hundred .mu.m between the host and the transplanted cells and may adversely effect diffusion kinetics. This diffusion-barrier may induce a significant "lag" time in detecting glucose levels within the blood that causes phase shifts in insulin secretion and therefore erratic regulation of blood levels glucose. Also, geometric constraints of the fibre technology result in very poor packing densities and may require up to several meters of transplanted islet encapsulated fibre.
One solution to these problems might be the use of the microencapsulation technique. In microencapsulation, cell clusters are immobilized in 500-600 .mu.m hydrogel microspheres. Typically the semipermeable membrane is formed at the microsphere surface. Various chemical systems have been used. In the most common form, the capsule membrane is formed by ionic or hydrogen bonds between two weak polyelectrolytes; typically an acidic polysaccharide, such as alginic acid, and a cationic polyaminoacid, such as polylysine. Practically, the entrapment of cells is obtained by the gelation of a charged polyelectrolyte induced by exposure to a multivalent counter-ion. A counter-polyelectrolyte is then interfacially adsorbed on the cell immobilization matrix. Microcapsules possess an ideal shape for diffusion. In vitro tests demonstrated that insulin release from microencapsulated islets was equivalent to that from unencapsulated cells. They are, however, mechanically fragile, particularly when polyelectrolytes are used. They are also chemically unstable as they rely only on ionic bonds for integrity, leading to rupture of the microcapsules after several weeks of implantation into the brain of non-human primates. Intraperitoneal implantation of such microcapsules has been reported to reverse diabetes in rodent experimental diabetes models and more recently in humans. The poor biocompatibility of the system raises however questions about its use in young diabetes patients. In an effort to correct the stability and biocompatibility issue, Sefton and collaborators are developing microcapsules based on the precipitation of an organic polymer solution around islet clusters. Problems of solvent toxicity and evenness of the permeability characteristics still hamper this approach. In general, the use of microcapsule systems in humans is limited by problems of long-term stability and process limitations to ensure a uniform thin coating on a large volume of islets.